Cal Tech Earthquake

Widely felt tremor not unprecedented

Q: How did the quake happen?

A: Virginia once was a heavy earthquake zone more than 200 million years ago, says Karen Fischer of Brown University, with once-active faults that have now cooled. "We are just seeing pressure build up and release on those scars," Fischer says. The pressure results from the spread of the Atlantic Ocean 's crust pushing against the East Coast 's crust. However, seismologists can't predict why or where quakes will happen on these faults.

Q: Why was this quake felt so widely?

A: Eastern North America rests on a fairly cold, connected stretch of continental crust. "So if you hit it, it tends to ring like a bell," says seismologist Peggy Hellweg of the University of California-Berkeley.

That's why the shakes from the quake travelled as far west as Chicago and as far north as Toronto.

Q: Why don't West Coast quakes travel this far?

A: "The western U.S. is more pieces all jumbled together and if you hit it, it tends to thunk like a pile of sand," Hellweg says. In California, shakes from a quake of this size are rarely felt more than 30 miles away.

Q: Where does this quake rank in the history of Mid-Atlantic quakes?

A: It is significant, but not unprecedented. A quake the size of Tuesday's last struck Virginia in 1897. Similar quakes have struck the wider region, most recently a magnitude-6.0 quake in Quebec on Nov. 25, 1986. And a magnitude-7.1 quake struck the same part of Canada in 1925.

But the Virginia quake is somewhat surprising because South Carolina and the New Madrid fault region near Memphis are seen as the hotspots for quakes in the eastern United States . The New Madrid earthquakes of 1811 and 1812, two magnitude-7.7 quakes sandwiching a 7.5 one, are the historical big ones of Mid-Atlantic quakes.

New ... - Caltech Media Relations - California Institute of Technology

PASADENA, Calif.— Like scars that remain on the skin long after a wound has healed, earthquake fault lines can be traced on Earth's surface long after their initial rupture. Typically, this line of intersection between the area where the fault slips and the ground is more complicated at the surface than at depth. But a new study of the April 4, 2010, El Mayor–Cucapah earthquake in Mexico reveals a reversal of this trend. While the fault involved in the event appeared to be superficially straight, the fault zone is warped and complicated at depth.

The study—led by researchers at the California Institute of Technology (Caltech) and documenting findings from the magnitude 7.2 event, which was centered in the Baja California state of Mexico—is available online in the journal Nature Geoscience .